With this addon I manage my local mail base. Without it, FF loses a great use for me and leaves me hanging. I go back to FF 56.0.2
Yes I have tried it and it seems good, but there are two issues:
1.- Nothing is as comfortable, simple and pleasant as managing mail in an FF tab.
2.- I do not know how I could pass to Thunderbird keeping the local mail database that I have in SimpleMail.
Thanks for your suggestion.